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  Java Practices -> Parse parameters into domain objects
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<div class='page-title'>Parse parameters into domain objects</div>

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<br>All HTTP request parameter names and values are entirely textual, and the Servlet Specification treats them as simple Strings.
The problem always arises in a web application of how to translate or parse such Strings into more meaningful objects in the problem domain. 
Typically, this means performing two kinds of operations:
<ol>
 <li>translating <tt>String</tt>s into <tt>Integer</tt>, <tt>BigDecimal</tt>, and so on
 <li>grouping such translated items into complete <a href='TopicActiond08d-2.html?Id=187'>Model Objects</a>
</ol>


<P>These operations will vary depending on the framework you're using. 

<P><b>Example</b>
<P>This example is taken from an application built with the <a href="TopicAction5f31-2.html">WEB4J</a> framework.
<br>
<PRE>

<span class='comment'>/** Build a Model Object from request parameters. */</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> RestoAction <span class='keyword'>extends</span> ActionTemplateListAndEdit {

  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> RequestParameter RESTO_ID = RequestParameter.withLengthCheck(<span class='literal'>"Id"</span>);
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> RequestParameter NAME = RequestParameter.withLengthCheck(<span class='literal'>"Name"</span>);
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> RequestParameter LOCATION = RequestParameter.withLengthCheck(<span class='literal'>"Location"</span>);
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> RequestParameter PRICE = RequestParameter.withLengthCheck(<span class='literal'>"Price"</span>);
  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> RequestParameter COMMENT = RequestParameter.withLengthCheck(<span class='literal'>"Comment"</span>);

  <span class='keyword'>protected</span> <span class='keyword'>void</span> validateUserInput() {
    <span class='keyword'>try</span> {
      ModelFromRequest builder = <span class='keyword'>new</span> ModelFromRequest(getRequestParser());
      fResto = builder.build(Resto.<span class='keyword'>class</span>, RESTO_ID, NAME, LOCATION, PRICE, COMMENT);
    }
    <span class='keyword'>catch</span> (ModelCtorException ex){
      <span class='comment'>//displays error message to end user
</span>      addError(ex);
    }    
  }
  
  <span class='comment'>//..many items elided
</span>
  <span class='comment'>//target Model Object, built from user input
</span>  <span class='keyword'>private</span> Resto fResto;
} 
</PRE>
<br>
In this case, the parsing of raw HTTP request parameters is shared between these classes :
<ul>
<li><a href='http://www.web4j.com/web4j/javadoc/hirondelle/web4j/request/RequestParameter.html'><tt>RequestParameter</tt></a> 
represents the raw underlying parameters accepted by the <tt>Action</tt></li>

<li><a href='http://www.web4j.com/web4j/javadoc/hirondelle/web4j/request/RequestParser.html'><tt>RequestParser</tt></a> 
defines default policies for translating <i>single</i> request parameters into <tt>String</tt>, <tt>Date</tt>,
<tt>Boolean</tt>, <tt>Integer</tt>, and <tt>BigDecimal</tt> objects, and <tt>Collection</tt>s thereof.</li>

<li>
<a href='http://www.web4j.com/web4j/javadoc/hirondelle/web4j/model/ModelFromRequest.html'><tt>ModelFromRequest</tt></a> 
translates <i>sets</i> of request parameters into Model Objects. While a <tt>RequestParser</tt> returns single "building block"
objects (<tt>Integer</tt>, <tt>Date</tt>, etc.),&nbsp; <tt>ModelFromRequest</tt> goes a step further, 
and returns an entire <a href='TopicActiond08d-2.html?Id=187'>Model Object</a>, which more or less encapsulates a 
number of "building block" objects.</li>

</ul>

<p>Validation and parsing of HTTP request parameters are related topics.
See the <a href="TopicAction2615-2.html">Repel invalid requests</a> topic for further information.
<br>
<br>

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<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicAction2615-2.html?Id=137'>Repel invalid requests</a> <br>
 
  
  <a href='TopicActionae2d-2.html?Id=139'>Command objects</a> <br>
 
  
  <a href='TopicActiond08d-2.html?Id=187'>Model Objects</a> <br>
 
  
  <a href='TopicAction5f31-2.html?Id=188'>A Web App Framework - WEB4J</a> <br>
 
</div>


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